Recently, for optical use and liquid crystal display use, an optical film that has excellent transparency and small in-plane retardation Re has been needed. In a VA mode liquid crystal cell, the refractive index in the direction vertical to the screen (thickness direction) is greater than the refractive index in the in-plane direction. Therefore, in order to improve the viewing angle in the VA mode, a film with a retardation where the refractive index in the thickness direction is smaller than the refractive index in the in-plane direction is required.
In order to obtain a film like the above-described film, a method for obtaining a retardation film is proposed in which a polycarbonate film is formed by solution casting, which is then sequentially and biaxially extruded in a state where it contains a certain amount of solvent (for example, Patent literature 1).
As another film formation method, there is a method according to which a film with small in-plane retardation is obtained by touch roller molding that uses an elastic metal roller.
According to a method for producing a thin film described in Patent literature 1, there is a part where the film does not make contact with the touch roller, and thus it is difficult to obtain a film with good appearance. Moreover, even the above-described touch roller molding that uses the elastic metal roller has difficulty in producing a film with sufficiently small in-plane retardation.
Furthermore, along with the recent expansion of the thin-type display market represented by liquid crystal televisions, there is an increasing need for a clearer image at a lower price. In order to realize this, various optical films, typically retardation films, are crucial.
Recently, along with the particularly increasing requirement for color reproducibility, there is a need for a polarizing plate protection film and a retardation film for liquid crystal display devices whose change in the retardation due to the wavelength of light is small. With respect to the conventionally utilized retardation films, a polycarbonate film as a polymer film with a positive intrinsic birefringence value, a styrene-based film for optical use as a film with a negative intrinsic birefringence value, and the like have been developed (Patent literature 2).
For such conventional films, however, change in the retardation due to the wavelength has not fully been considered. In addition, the thickness-direction retardation Rth as well has not been considered upon designing.
Patent literature 3 discloses, for the purpose of enhancing the image quality of a liquid crystal display device, use of a multilayer film upon making a so-called reverse wavelength dispersion-type film in which the retardation value becomes larger as the wavelength becomes longer. Specifically, it is a multilayer film made from a piece of polymer-blend film of polyphenylene oxide and polystyrene, which is obtained by layering a retardation plate that has a wavelength band with a positive retardation value (in-plane retardation Re) and a wavelength band with a negative retardation value at a wavelength of 400-700 nm, with a retardation film made from, for example, polycarbonate, in which the retardation value is positive or negative at a wavelength of 400-700 nm.
Even in Patent literature 3, however, the thickness-direction retardation Rth was not considered.    Patent literature 1: Japanese Patent Laid-Open No. 2004-149639    Patent literature 2: Japanese Patent Laid-Open No. H03(1991)-24502    Patent literature 3: Japanese Patent Laid-Open No. 2001-42121